London's urban heat island effect, which keeps night-time temperatures in the capital warmer than in surrounding rural areas, may have been underestimated by up to 45 per cent.

The heat can pose serious health risks, particularly for the elderly and very young.

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Until now, the effect has been measured by calculating the difference between temperatures in St James' Park in the city, and Wisley - a rural site just outside the M25.

But new research, published in the journal Science of the Total Environment, found night time temperatures in parks can be up to 4°C cooler than in the streets nearby. So the St James' Park measurements might have dramatically underestimated the urban heat island in the capital.

'In the summer time, built-up areas effectively act like a storage heater,' says Dr Kieron Doick of Forest Research, the research agency for the UK Forestry Commission, who led the research. 'They store up heat during the day and release it at night.'